The morphology, composition, and function of struts that interconnect the lateral surfaces of cardiomyocytes were examined in the hearts of rats and hamsters. Methods included brightfield and fluorescent light microscopy, secondary and backscatter scanning electron microscopy, and transmission electron microscopy in conjunction with silver stain, cationic dye, and antibody to type-I collagen. These studies reveal a twisted, beaded appearance and a complex substructure of collagen fibrils embedded in a ground substance that has a positive reaction with cationic dye. A hierarchy of patterns of branching and attachment was seen among intercellular struts ranging in diameter from 0.1 micron to several micron. The hypothesis that struts tether not only the surfaces but the contractile lattices of laterally adjacent myocytes is supported by the following: (a) the attachments of struts to the collagen weave of the sarcolemma, often lateral to the level of Z bands, (b) the presence of collagen type I in a composite material arrangement, (c) the relative dispositions and configurational changes of struts and myocyte surfaces in various physiological states and induced, non-physiological perturbations of cardiac muscle, (d) the corrugated sarcolemmas with infoldings near Z bands, and (e) the continuity of intracellular filaments from Z bands to the inner aspect of the sarcolemma in relaxed and contracted myocytes. Implications of struts acting as tethers and sites for storage of energy in the motions of myocytes during the cardiac cycle are discussed.